Module file

Unpack the rar file into an arbitrary directory, lets say it is called c:\data\mayaToMantra. In this directory you should now see several directorys and a file called mayaToMantra.mod

Edit this file with a text editor. There is one line called

+ mtm 0.1 C:\mayaToMantra

modify this line and set the path to the directory which contains the mayaToMantra.mod, in our case c:\data\mayaToMantra. Now you have:
+ mtm 0.1 c:\data\mayaToMantra
Save the file.

Maya env file

Open the maya.env file in your home/documents/maya/2012-x64 directory. Here you need at least two entries which tells the plugin where to find its ressources. This is simply the path where the module is located here again - c:\data\mayaToMantra. Now write this line into the maya.env file:
MTM_HOME= c:\data\mayaToMantra
And it is necessary to tell maya where to find the module file wich we edited above so add:
Save and start maya. Now you should see the plugin in the plugins list and you can load it.
To test the installation, you can load one of the example scenes in example project in the mayaToMantra directory. There are a few scenes that show subdiv rendering, instances, dof.

The standalone tool HoudiniObjectSaver.exe needs some houdini libraries. This means the 'H' variable has to be set correctly. You can do it simply in the same env file with:
H=C:\Program Files\Side Effects Software\Houdini 11.1.22
Important: The installed Houdini version have to match the one used for compiling the standalone. At the moment it is Houdini 11.1.22. Other versions will cause the converter to fail.

Take care that maya is able to find the HoudiniObjectSaver.exe. You can simply add the path to the maya.env file:

Renderer Description File

Every renderer that can be used with the render -r xxx command needs a rendererDescription XML file. These files are located in the bin/rendererDescription directory of the current maya installation.
For mayaToMantra there exists such an XML file as well. It is located in the ressources directory of the mayaToMantra installation directory. There are two possibilities to make sure that maya will find it for rendering.
The first one is to place this file in the maya/bin/rendererDescription subfolder of your maya installation. That should work. However this means that every time you download a new mayaToMantra installation, you will have to replace this file. Alternativly you can create an environment variable called  MAYA_RENDER_DESC_PATH which points to your mayaToMantra installation path like this:
Then you can use the command:
render.exe -r mantra -s 10 -e 100